Different types of nonresponse threaten the validity of longitudinal studies: First, the initial nonresponse during the recruitment in the base-line survey; Second, successive dropout at each time point. In this entry, specific attention is given to dropout and strategies to limit the problem. Dropout in longitudinal surveys has three separate sources: failure to locate research participants, failure to contact research participants, and failure to achieve cooperation. Effective measures are described to reduce nonresponse. To limit nonresponse, a total design approach is advocated with specific attention to each source. That is to limit both noncontact (i.e., failure to locate and subsequent failure to contact a located research participant), and noncooperation. Keywords: panel; nonresponse; attrition; prevention